Method and apparatus reducing off track head motion due to disk vibration in a hard disk drive through the head gimbal assembly

ABSTRACT

Improved head gimbal assemblies reducing TMR (Track Mis-Registration) in a hard disk drive are provided. These head gimbal assemblies are as mechanically simple as contemporary head gimbal assemblies, support parallel flying sliders over flat disk surfaces, and reduce TMR induced by disk vibration. They are easier to build, more reliable, and cost less to make, than other known approaches at comparable track densities and rotational rates. The improved head gimbal assemblies include three sets of mechanisms for moving the slider parallel the disk surface, when the disk surface is flat, and radially moving the slider toward the track, when the disk surface is bent. The first and third mechanisms as well as the second and third mechanisms can be used together in a head gimbal assembly. An improved and distinctive servo-controller scheme resulting in an overall improvement in PES performance, particularly when applied to hard disk drives employing the invention&#39;s TMR reduction mechanisms. The servo-controllers trade off gain in the disk vibration frequency range, in favor of, increased rejection of low frequency disturbances. This leads to the lowest PES statistics, when applied to hard disk drives with the TMR reduction mechanisms of the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to head gimbal assemblies and servocontroller of a hard disk drive.

2. Background Information

Hard disk drives contain a plurality of magnetic heads that are coupledto rotating disks. The heads write and read information by magnetizingand sensing the magnetic fields of the disk surfaces. There have beendeveloped magnetic heads that have a write element for magnetizing thedisks and a separate read element for sensing the magnetic field of thedisks. The read element is typically constructed from amagneto-resistive material. The magneto-resistive material has aresistance that varies with the magnetic fields of the disk. Heads withmagneto-resistive read elements are commonly referred to asmagneto-resistive (MR) heads.

Each head is embedded in a slider, which is attached to a flexure arm tocreate a subassembly commonly referred to as a head gimbal assembly(HGA). The HGA's are attached to an actuator arm. The actuator arm has avoice coil motor that can move the heads across the surfaces of thedisks.

Information is stored in radial tracks that extend across the surfacesof each disk. Each track is typically divided up into a number ofsegments or sectors. The voice coil motor and actuator arm can move theheads to different tracks of the disks and to different sectors of eachtrack.

A suspension interconnect extends along the length of the flexure armand connects the head to a preamplifier. The suspension interconnecttypically comprises a pair of conductive write traces and a pair ofconductive read traces. One pair of traces, such as the read traces,extend down one side of the flexure arm to the head and the remainingpair of traces extends down the other side of the flexure arm to thehead.

The Tracks Per Inch (TPI) in hard disk drives is rapidly increasing,leading to smaller and smaller track positional tolerances. The trackposition tolerance, or the offset of the read-write head from a track,is monitored by a signal known as the head Positional Error Signal(PES). Reading a track successfully usually requires minimizingread-write head PES occurrences. The allowable level of PES is becomingsmaller and smaller. A substantial portion of the PES is caused by diskvibration.

Track Mis-Registration (TMR) occurs when a read-write head tends to losethe track registration. This occurs when the disk surface bends up ordown. TMR is often a statistical measure of the positional error betweena read-write head and the center of an accessed track. Bending isdefined in terms of bending modes. For a positive integer k, a bendingmode of (k,0) produces k nodal lines running through the disk surfacecenter, creating k peaks and k troughs arranged on the disk surface.Bending mode (0,0) produces no nodal lines, either the entire disk isbent up or bent down.

Two basic prior art approaches are known to lower the TrackMis-Registration (TMR) due to disk vibration. One approach uses headgimbal assemblies providing a radial motion capability. The otherapproach alters the servo-controller to reduce TMR.

In the first approach, a head gimbal assembly, including a biased loadbeam, creates a roll center (also known as a dimple center), whichprovides a radial motion capability as the load beam moves verticallydue to disk vibration. This allows sliders to move in a radial directionas well as in a vertical direction with respect to the disks, reducingoff-track motion due to disk vibration.

The first approach has some problems. An air bearing forms between theslider face and the disk surface. The slider face is tilted near thedisk surface when it is flat. The air bearing becomes non-uniform whenthe disk surface is flat, adding new mechanical instabilities into thesystem.

One alternative prior art head gimbal assembly provides a slider mountedso that it pivots in the radially oriented plane about the effectiveroll axis, which is located within the disk. This scheme does not causea non-uniform air bearing when the disk surface is flat. However, theway the effective roll axis is placed inside the disk requires a morecomplex mechanical coupling between the slider support assembly and theslider. This complex mechanical coupling may have a greater probabilityof mechanical failure, tending to increase manufacturing expenses and toreduce hard disk drive life expectancy.

The second prior art approach to lowering TMR due to disk vibrationalters the servo-controller. These servo controllers favor optimizationof PES in the disk vibration range without regard for strengtheningrejection of low frequency disturbances. The disk vibration range willbe considered to include frequencies between about 1K Hz and about 4KHz. Low frequency disturbances will be considered to include at leastthe frequencies between about 0 Hz and about 800 Hz.

Accordingly, there exists a need for head gimbal assembly mechanismsproviding a stable air bearing, able to follow a track when a disksurface bends, which are easy and reliable to manufacture. There existsa need for servo controllers optimizing PES in the disk vibration rangeand taking into account potential advantages from strengthened rejectionof low frequency disturbances.

BRIEF SUMMARY OF THE INVENTION

The present invention includes improved head gimbal assemblies, whichaddress TMR. These head gimbal assemblies are as mechanically simple ascontemporary head gimbal assemblies, support parallel flying slidersover flat disk surfaces, and reduce TMR induced by disk vibration. Theymay be easier to build, more reliable, and cost less to make, than otherknown approaches, at comparable track densities and rotational rates.The improved head gimbal assemblies include mechanisms for moving theslider parallel the disk surface, when the disk surface is flat, andradially moving the slider toward the track, when the disk surface isbent, so that the head can more closely follow the track.

In a first set of mechanisms, the actuator arm moves by lever actionthrough a principal axis, with the slider aligned at a bias angle andthe slider face parallel to the flat disk surface. The lever actioncauses the slider to move radially toward the track, when the disksurface is bent.

In a second set of mechanisms, the actuator arms couple to load beamsvia two fingers. The first finger flexes differently from the secondfinger when the disk surface is bent. The fingers are constructed sothat, when the disk surface is bent, the slider is moved radially towardthe track.

A third set of mechanisms move the actuator arm by lever action throughthe principal axis. The actuator holds the slider parallel to the disksurface, when it is flat. The slider is mounted by a flexure at a secondbias angle to the principal axis. The flexure responds as the disksurface bends through the second bias angle, causing the slider to moveradially toward the track.

The present invention provides head gimbal assemblies incorporatingmechanisms of the first set operating with mechanisms of the third set.The invention alternatively provides head gimbal assembliesincorporating mechanisms of the second set operating with mechanisms ofthe third set.

The present invention provides a distinctive servo-controller schemeresulting in overall improvement in PES performance, particularly whenapplied to hard disk drives employing the invention's TMR reductionmechanisms.

The servo-controllers trade off gain in the disk vibration frequencyrange in favor of increased rejection of low frequency disturbances.This leads to the lowest PES statistics, when applied to hard diskdrives with the TMR reduction mechanisms of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the present invention, which are believed tobe novel, are set forth with particularity in the appended claims. Thepresent invention, both as to its organization and manner of operation,together with further objects and advantages, may best be understood byreference to the following description, taken in connection with theaccompanying drawings, in which:

FIG. 1 is a simplified schematic of the relationship between a principalaxis of the actuator arm, head gimbal assembly, slider, and radialvector from the center of the spindle hub;

FIG. 2 is a simplified schematic of a disk drive controller controllinga hard disk drive;

FIG. 3A is a cross section view through a disk of a first inventivemechanism operating when the disk surface bends down;

FIG. 3B is a radial-directional view through the disk of the firstinventive mechanism, when the disk surface bends down;

FIG. 3C is a cross section view through the disk of the first inventivemechanism operating when the disk surface bends up;

FIG. 3D is a radial-directional view through the disk of the firstinventive mechanism operating when the disk surface bends up;

FIGS. 4A to 6A are top views of head gimbal assemblies of the first andsecond inventive mechanisms;

FIGS. 6B to 6E are views of head gimbal assemblies of the secondinventive mechanism;

FIG. 7A is a top view of a head gimbal assembly of the first and secondinventive mechanisms;

FIGS. 7B to 8D are views of a head gimbal assembly of the thirdinventive mechanism;

FIGS. 9A, 9C, and 9E are side views of the radial head motions for headgimbal assemblies of FIG. 8D, resulting from the bending motion of theflexure gimbal induced by disk axial vibration;

FIGS. 9B, 9D, and 9F are top views of the radial head motions for headgimbal assemblies of FIG. 8D, resulting from the bending motion of theflexure gimbal induced by disk axial vibration;

FIGS. 10A, 10C, and 10E are side views of the radial head motions forhead gimbal assemblies of FIG. 7B, resulting from the bending motion ofthe flexure gimbal induced by disk axial vibration;

FIGS. 10B, 10D, and 10F are top views of the radial head motions forhead gimbal assemblies of FIG. 7B, resulting from the bending motion ofthe flexure gimbal induced by disk axial vibration;

FIGS. 11A and 11B are cross section and radial-directional views,through the disk, of the mechanisms of FIGS. 7B through 8D operatingwhen the disk surface bends down;

FIGS. 11C and 11D are cross section and radial-directional views,through the disk, of the mechanisms of FIGS. 7B through 8D operatingwhen the disk surface bends up;

FIG. 12 is a graph of the results of the bending slope per unit of axialdisplacement for four common bending modes for various radial positionsand ID through OD;

FIGS. 13A, 13B, and 13C are graphs summarizing results regarding thepower spectral density function in terms of axial vibration frequencyversus displacement in meters at ID, at MD, and at OD, respectively;

FIG. 14A is the geometric analysis used for the roll bias angle formulafor the mechanisms of FIGS. 7B to 8D when the disk surface bends down asin FIG. 11A;

FIG. 14B is the geometric analysis used for the roll bias angle formulafor the mechanisms of FIGS. 7B to 8D, when the disk surface bends up asin FIG. 11C;

FIG. 14C is the geometric analysis used for the roll bias angle formulafor the roll center mechanisms of FIGS. 4A to 7A when the disk surfacebends down as in FIG. 8A;

FIG. 14D is the geometric analysis used for the roll bias angle formulafor the roll center mechanisms of FIGS. 4A to 7A when the disk surfacebends up as in FIG. 9A;

FIGS. 15A and 15B are graphs of the results of the bending angle versusthe roll bias angle at OD when the disk is respectively bent down andbent up;

FIG. 16A is a graph summarizing the spectral density of NRRO PES/Trackpitch measure in percent versus vibrational frequency for a disk-headgimbal assembly with a roll bias angle of zero degrees, which isstandard in conventional hard disk drives;

FIGS. 16B and 16C are graphs summarizing the spectral densities of NRROPES/Track pitch measure in percent versus vibrational frequency for adisk-head gimbal assembly with a roll bias angle of one degree and oftwo degrees, respectively;

FIG. 17 is a graph of a weight function for PES feedback in thefrequency domain trading servo controller gain in disk vibrationfrequency range for increased rejection of low frequency disturbances;

FIG. 18 is a graph of the error sensitivity function of the originalservo controller and the modified servo controller as derived from FIG.17;

FIG. 19 is a graph of the mechanical disturbance spectra of the in-planetorque disturbance spectrum and out-of-plane disk disturbance spectrum;and

FIGS. 20 and 21 are graphs of the results regarding a conventionaldisk-head gimbal assembly interface, compared to a head-gimbal assembly,with a two degree roll bias angle, operated with the modifiedservo-controller.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description is provided to enable any person skilled inthe art to make and use the invention and sets forth the best modespresently contemplated by the inventors of carrying out the invention.Various modifications, however, will remain readily apparent to thoseskilled in the art, since the generic principles of the presentinvention have been defined herein.

Disclosed are improved head gimbal assemblies addressing TrackMis-Registration (TMR). These head gimbal assemblies may be asmechanically simple as contemporary head gimbal assemblies, supportparallel flying sliders over flat disk surfaces, and reduce TMR inducedby disk vibration. They are easier to build, more reliable, and costless to make, than other known approaches at comparable track densitiesand rotational rates. The improved head gimbal assemblies includeimproved suspensions or mechanisms for moving the slider parallel to thedisk surface, when the disk surface is flat, and radially moving theslider toward the track, when the disk surface is bent.

Referring to the drawings, more particularly by reference numbers, FIG.1 shows an example actuator arm assembly pivoting about the actuatoraxis 40, changing the angle between the radial vector 112 and theactuator principal axis 110. The actuator arm assembly includes theactuator arm 50 coupled to head gimbal assembly 60, which is coupled toslider 100. Typically, the actuator arm assembly will rotate throughvarious angles between a furthest inside position of the disk and thefurthest outside position on the disk. Test data and analyses areprovided for three regions of the disk. These are designated ID(corresponding to the furthest inside position), MD (a middle positionwhere radial vector 112 is approximately at a right angle with 110), andOD (the furthest outside position).

In FIG. 1, an X axis extends along the principal axis 110 of theactuator arm, and a Y axis intersects the X axis at essentially actuatorpivot 40. When the actuator arm 50 positions the slider 100 so that theread-write head is at MD, the radial vector 112 is nearly parallel tothe Y axis. Track 18 is shown near MD, but tracks exist from ID to OD,across the disk surface 12.

The hard disk drive may include a plurality of actuator arms and headsliders located adjacent to the disks all controlled by the same voicecoil motor. The heads may have separate write and read elements, thatmagnetize and sense the magnetic field of the disks.

FIG. 2 shows a schematic of an example controller system for a diskdrive. The controller system includes a voice coil 32 coupled to amagnet assembly to create a voice coil motor. Providing a current to thevoice coil 32 creates a torque that swings the actuator arm 50,contained in the actuator assembly 30. Moving the actuator arm 50 movesthe actuator arm assembly, which moves the heads across the surfaces ofthe disk 12.

The hard disk drive may further include a disk drive controller 1000. InFIG. 2, disk drive controller 1000 communicates with an analogread-write interface 220, which in turn communicates the resistivityfound in the spin valve within read-write head to controller 1000. Theanalog read-write interface 220 frequently includes a channel interface222 communicating with a pre-amplifier 224. The channel interface 222receives commands from the embedded disk controller 1000, setting theread_bias and write_bias. The analog read-write interfaces 220 mayemploy either a read current bias or a read voltage bias. For example,the resistance of the read head is determined by measuring the voltagedrop (V_rd) across the read differential signal pair (r+ and r−), basedupon the read bias current setting read_bias, using Ohm's Law.

In FIG. 2, the channel interface 222 provides a Position Error Signal(PES) to the servo controller 240, which controls voice coil 32 to keepthe read-write head close enough to access a data track (such as track18 of FIG. 1).

The invention includes three approaches for addressing the TMR problemby moving the slider face parallel to the disk surface with respect tothe track when the disk surface is flat, and a moving the slider towardthe track when the disk surface is bent. Each of these approaches forreducing TMR may be used individually or in combination with otherapproaches discussed herein or elsewhere.

Actuator assemblies using example mechanisms employing the firstapproach are seen in FIGS. 4A to 6A and 7A. In these example mechanismsthe actuator arm moves by lever action through a principal axis, withthe slider aligned at a bias angle and the slider face parallel to theflat disk surface. The lever action causes the slider to move radiallytoward the track, when the disk surface is bent.

In addition to the example mechanisms employing the first approach,FIGS. 4A to 6A also show actuators with example mechanisms employing thesecond approach. FIGS. 6B to 6E show actuator assemblies which use onlythe second approach to reducing TMR. As will be explained further below,the example mechanisms employing the second approach include two fingerscoupling the actuator arm to the load beam, where the fingers flexdifferently from each other when the disk surface is bent. Thedifference in the response of each finger to the bending of the disksurface is designed to cause the slider to move radially toward thetrack when the disk surface bends.

FIGS. 7B to 8D show example mechanisms employing the third approach toreducing TMR. In these mechanisms, the slider is mounted by a flexure ata second bias angle to the principal axis. The actuator holds the sliderparallel to the disk surface, when it is flat. The flexure responds asthe disk surface bends through the second bias angle, causing the sliderto move radially toward the track.

In operation, these mechanisms provide a suspension with a roll centermovable slider which allows the read-head to follow a track as the discsurface bends. This process is seen more clearly in reference to FIGS.3A to 3D which show schematically the head-disk interface dynamics.FIGS. 3A and 3B show cross section and radial-directional views throughthe disk 14 of the inventive mechanisms discussed above operating whenthe disk surface 12 bends down, and FIGS. 3C and 3D show cross sectionand radial-directional views through the disk 14 of a first inventivemechanism operating when the disk surface 12 bends up.

Throughout this document, the read-write head position when the disksurface is flat is denoted A0, and when bent is denoted A1. The trackposition when the disk surface is flat is denoted B0, and when bent, isdenoted B1. δ refers to the amount of this off-track movement, or thedistance between A1 and B1. δ1 refers to the distance between A0 and A1.δ2 refers to the distance between B0 and B1.

This head-disk surface interface allows the use of the formula δ=δ1+δ2,for the motion of FIGS. 3A and 3C, given that A0−B0 and A1−B1 areessentially 0, because $\begin{matrix}{{{Distance}\left( {{A1} - {B1}} \right)} = {{Distance}\left( {{A1} - {A0} + {A0} - {B1} + {B0} - {B0}} \right)}} \\{= {{Distance}\left( {\left( {{A1} - {A0}} \right) + \left( {{B0} - {B1}} \right) + \left( {{A0} - {B0}} \right)} \right)}} \\{\approx {{Distance}\left( {\left( {{A1} - {A0}} \right) + \left( {{B0} - {B1}} \right)} \right)}} \\{{\delta \approx {{\delta 1} + {\delta 2}}} = 0.}\end{matrix}$

In FIGS. 3A and 3C, δ2=td/2*θ=−δ1.

Each of the previously mentioned approaches will be discussed in moredetail below.

FIGS. 4A to 6A, and 7A, show several means for moving the sliderparallel to a disk surface. Including moving the actuator arm by a leveraction through the principal axis with the slider aligned at a biasangle 710, when the slider face is essentially parallel to the disksurface.

With respect to the track when the disk surface is flat, FIGS. 4A to 6A,and 7A, show the means for radially moving the slider. Each of whichincludes a means for the lever action to cause the slider to moveradially bent. More specifically, the lever action through the principalaxis at the bias angle 710, causes the radial motion of the slider. Inthese Figures, the dynamics of the head-disk interface determine thebias angle 710, which is preferably between plus and minus 10 degreesfrom zero.

In FIG. 4A, a head gimbal assembly includes a suspension having biasangle 710, attached by connection beam 82 to extended base plate 84. Thesuspension mounts on the actuator arm, at bent edge 700 of extended baseplate 84 and at bent edge 702 of load beam 80.

In FIG. 4B, a head gimbal assembly includes a suspension having biasangle 710, attached to base plate 70. It mounts on the actuator arm, atbent edge 704 of base plate 70 and at bent edge 700 of load beam 80.

In FIG. 4C, a head gimbal assembly includes a suspension having biasangle 710, attached by connection beam 82 to extended base plate 84.This is mounted on the actuator arm, at the bent edge 700 of theextended base plate 84.

In FIG. 4D, a head gimbal assembly includes a suspension having biasangle 710, attached to base plate 70, which is mounted on the actuatorarm, at bent edge 704 of base plate 70.

FIGS. 4A to 6B, and 7A, show the second inventive mechanism for reducingTMR. This includes the means for moving the actuator arm coupled to theload beam via two fingers. The first finger flexes differently from thesecond finger when the disk surface is bent. The first finger flexesdifferently from the second finger, causing the slider to move radiallywith respect to the track when the disk surface is bent.

In particular, FIGS. 6B to 6E show embodiment of the second mechanism,which do not involve a bias angle 710 as shown in FIGS. 4A to 6A and 7A.FIG. 6B shows a top view of a head gimbal assembly, based upon thedifference in the connection beam fingers 82-A and 82-B connecting up83-A and down 83-B, respectively. Finger 82-A flexes differently fromfinger 82-B, causing slider 100 to move radially with respect to thetrack, as disk surface 12 is bent. FIG. 6C shows fingers 82-A and 82-Bconnecting up and down on the upper connection beam 86 and lowerconnection beam, as in FIG. 6E. FIGS. 6D and 6E show side views of thehead gimbal assembly of FIG. 6B when the thickness of the base plate 70and load beam 80 differ, or are the same, respectively. Extended baseplate 84, finger 82-A, and upper connection beam 86 are preferably madefrom one sheet of metal, preferably stainless steel.

FIG. 7A shows a top view of suspension 80 attached to base plate 70using a cutout 720 to create fingers 80-A and 80-B, which together formbias angle 710. The bias angle 710 can be determined by the dynamics ofthe head-disk interface, the shape of cut-out 720, and the stiffness ofload beam 80.

FIGS. 7B to 8D show examples of a third inventive mechanisms forreducing TMR. This includes moving the actuator arm by the lever actionthrough the principal axis when the slider is parallel to the disksurface. The slider is mounted on a flexure at a second bias angle 710to the principal axis. The means for radially moving the slider includesthe flexure responding as the disk surface bends through the second biasangle, causing the slider to move radially toward the track.

FIG. 7B shows a top view of the suspension attached to base plate 70,with two points 722 and 724 welding flexure 90 to load beam 80,providing bias angle 710. Flexure 90 attaches to both slider 100 and toload beam 80. The number of welding points close to the slider ispreferably at least two. If the line between welding points is notperpendicular to the principal axis 110, then the trajectory of bendingmotion of the flexure induced by disk axial vibration will be on atilted bending line. It is sometimes preferred that the line, betweenthe welding points 722 and 724, is not perpendicular to the principalaxis 110.

FIGS. 9A to 9F show radial head motions for the head gimbal assembly ofFIG. 8D, due to the bending motion of the flexure gimbal induced by diskaxial vibration. In FIGS. 9B, 9D, and 9F, the trajectory of head, movesabout the tilted bending line 740. This line is preferably in a range ofplus or minus 10 degrees of arc from zero.

FIGS. 10A to 10F show radial head motions for the head gimbal assemblyof FIG. 7B, according to the bending motion of the flexure gimbalinduced by the disk axial vibration. In FIGS. 10B, 10D, and 10F, thetrajectory of head, moves about the tilted bending line 740. This lineis again, preferably, in a range of plus or minus 10 degrees of arc fromzero.

Embodiments of a third set of example mechanisms built according to theinvention are shown in FIGS. 7B through 8D. FIGS. 11A to 11D showvarious views of this set of mechanisms.

FIG. 11A shows a cross section view, through disk 14 when the disksurface 12 bends down 12-A. FIG. 11B shows a radial-directional viewwhen disk surface 12 bends up 12-B.

FIG. 11C shows a cross section view when the disk surface 12 bends up12-B. FIG. 11D shows a radial-directional view when the disk surface 12bends up 12-B.

It is reasonable to use the formula δ=δ1−δ2=0, for the motion of FIGS.11A and 11C, since A0−B0 and A1−B1 are essentially 0, leading, as inFIG. 3C, to δ2=td/2*θ=−δ1.

FIG. 12 shows the results of the bending slope per unit axialdisplacement, for various radial positions for four common bending modesfor ID through OD. The horizontal axis indicates radial position interms of meters. The vertical axis indicates the bending slope per unitaxial displacement in terms of 1/meter. Trace 800 indicates the resultsfor bending mode (3,0). Trace 802 indicates the results in bending mode(2,0). Trace 804 indicates the results in bending mode (1,0). Trace 806indicates the results in bending mode (0,0).

FIGS. 13A, 13B, and 13C summarize results regarding the power spectraldensity function in terms of axial vibration frequency versusdisplacement in meters at ID, at MD, and at OD, respectively.

In FIGS. 12 through 13C, the experimental hard disk drive was rotatingat 7200 RPM.

FIGS. 14A and 14B show the geometric analysis used for the bias angle710 formula for the mechanisms of FIGS. 7B to 8D when the disk surfacebends down as in FIG. 11A and bent up as in FIG. 11C. In these Figures:

-   -   A refers to the upper center point of slider 100 for the        stationary state.    -   C refers to upper center point of slider 100 for the disk        spinning state.    -   c refers to a deformed track 18 on the spinning disk surface 12.    -   ts refers to the thickness of slider 100.    -   td refers to the thickness of disk 14.    -   θ refers to the disk bending angle.    -   r refers to the radius of the disk, which is preferably 45 mm.    -   φ refers to the roll bias angle, which is portrayed in FIGS. 7B        to 8D by reference number 710.

In FIGS. 14A to 14D, the bending angle per unit of axial vibration at rwas experimentally determined to be

-   -   75/m for bending mode (3,0),    -   60/m for bending mode (2,0),    -   50/m for bending mode (1,0), and    -   40/m for bending mode (0,0).

The bias angle 710 of the earlier Figures is the roll bias angle φ ofFIGS. 14A-16C. In FIG. 14A, the roll bias angle φ=arc cos( (a−b cosθ)/c) for the disk bending down. In FIG. 14B, the roll bias angle φ=arccos( (b cos θ−d)/c) for the disk bending up. This leads to φ=1.2 degreesof arc at the Outside position of Disk OD, with r=45 mm. In FIGS. 14Aand 14B, the radial motion of slider 100 is about h=b* sin θ=(ts+td/2)sin θ.

FIG. 14C shows the geometric analysis of the roll bias angle 710 formulafor the roll center mechanisms of FIGS. 4A to 6A, and 7A, when the disksurface bends down as in FIG. 8A.

FIG. 14D shows the geometric analysis of the roll bias angle 710 formulafor the roll center mechanisms of FIGS. 4A to 6A, and 7A, when the disksurface bends up as in FIG. 9A.

In FIGS. 14C and 14D:

-   -   Ar refers to the roll center for the stationary state.    -   Cr refers to roll center for the disk spinning state.    -   c refers to a deformed track 18 on the spinning disk surface 12.    -   ts refers to the thickness of slider 100.    -   td refers to the thickness of disk 14.    -   θ refers to the disk bending angle.    -   r refers to the radius of the disk, which is preferably 45 mm.    -   φ refers to the roll bias angle, which is portrayed in FIGS. 4A        to 6A, and 7A by reference number 710.

In FIG. 14C, the roll bias angle φ=arc cos( (ar−br cos θ)/cr) for thedisk bending down. In FIG. 14D, the roll bias angle φ=arc cos( (br cosθ−dr)/cr) for the disk bending up. This leads to φ=1.6 degrees of arc atthe Outside position of Disk OD, with r=45 mm.

Drive level experiments were conducted on two types of production harddisk drives with the roll biased load beam built to move the roll centerradially as shown in FIG. 5A. The hard disk drives operated at 56,000TPIat 7200 RPM and at 93,000 TPI at 7200 RPM. Both types of hard diskdrives were able to move the roll center radially with the biased loadbeam.

Skew angles as used herein refer to the angular difference from theperpendicular of the principal axis 110 of the actuator with respect tothe tangent of the track 18. In the experimental hard disk drives, theskew angle at OD is about 13.1 degrees arc, at MD about −5 degrees arc,and at ID about −18 degrees arc. Please refer to FIG. 1 for anillustration of these positions.

FIGS. 15A and 15B show the results regarding the bending angle versusthe roll bias angle at OD when the disk bends down and up, respectively.The vertical axes represent the roll bias angle φ (710) in terms ofdegrees arc. The horizontal axes represent the bending angle in units of10⁻⁵ degrees of arc.

The roll biased load beam acts to attenuate several peaks related todisk modes in the spectrum of the non-repeatable run-out (NRRO) PESsignal, shown in FIGS. 16A-16C. In addition, the repeatable run-out(RRO) level is also attenuated, because of the attenuation of the NRROin the writing of the servo track. The results in the following tableare for tracks near OD. Roll bias angle Std-RRO (%) Std-NRRO (%)Std-Total (%) Standard (0) 1.582 1.855 2.441 One degree 1.484 1.4652.070 Two degrees 1.328 1.406 1.936

FIG. 16A summarizes the spectral density 820 of NRRO PES/Track pitchmeasure in percent versus vibrational frequency for a disk-head gimbalassembly with no roll bias angle, which is standard in conventional harddisk drives. FIGS. 16B and 16C summarize the spectral densities 822 and824 of NRRO PES/Track pitch measure in percent versus vibrationalfrequency for a disk-head gimbal assembly with a roll bias angle of onedegree and of two degrees, respectively.

In FIGS. 16A to 16C, the vertical axis represents NRRO per track pitchas a percentage. The horizontal axis represents vibrational frequency inHerz.

Reference labels 1B and 1F represent the backward frequency and theforward frequency associated with bending mode (1,0).

Reference labels 2B and 2F represent the backward frequency and theforward frequency associated with bending mode (2,0).

Reference labels 3B and 3F represent the backward frequency and theforward frequency associated with bending mode (3,0).

Reference labels 4B and 4F represent the backward frequency and theforward frequency associated with bending mode (4,0).

TMR is further reduced by reconfiguring the servo controller 240 of FIG.2 for roll biased head gimbal assemblies, by sacrificing gain in thedisk vibration region of the spectrum and increasing suppression in thelow frequency region. In conventional hard disk drives, gain in the diskvibration frequency range cannot be sacrificed.

FIG. 17 shows a weight function 840-848 for PES feedback in thefrequency domain. The servo controller gain in disk vibration frequencyrange 842 to 846 is traded for increased rejection of low frequencydisturbances 840. FIG. 17 was derived by a random search method with theweighted function for PES in the neighborhood ranges, 840-848.

FIG. 18 shows the error sensitivity function of the original servocontroller 850 and the modified servo controller 852 as derived fromFIG. 17. The vertical axis represents the error sensitivity in decibels.The horizontal axis represents the disk vibrational frequency in Herz.

F1 indicates a definition of disk vibration frequency range, from 1K Hzto 3K Hz. F2 indicates an alternative definition, from 800 Hz to 4K Hz.

F3 indicates a definition of low frequency range from 17 Hz to 800 Hz.F4 indicates an alternative definition, from 0 Hz to 800 Hz.

The preferred definition of disk vibration frequency range may varyamong hard disk drives, possibly including higher and/or lowerfrequencies.

The preferred definition of low frequency range may vary among hard diskdrives, possibly including higher and/or lower frequencies.

FIG. 19 shows the mechanical disturbance spectra. Trace 860 representsthe in-plane torque disturbance spectrum. Trace 862 represents theout-of-plane disk disturbance spectrum. References labels 3B, 3F, 4B,and 4F represent backward and forward resonance of bending modes (3,0)and (4,0). The vertical axis represents displacement on a logarithmicscale. The horizontal axis represents mechanical vibration in Herz.

FIGS. 20 and 21 show the results regarding a conventional disk-headgimbal assembly interface 870, compared to a head-gimbal assembly with atwo degree roll bias angle, operated with the modified servo-controller872. Trace 870 represents a conventional disk and head gimbal assemblyinterface with a total NRRO PES of 2.578%. Trace 872 represents theexperimental hard disk drive with a roll bias angle of two degreesoperated by the modified servo-controller, having a total NRRO PES of1.621%. This is a significant reduction in TMR, as measured by PES.

The invention includes applying this servo-controller scheme to any TMRreducing mechanism showing favorable results when trading off gain atthe disk vibration frequency range in favor of increased rejection oflow frequency disturbances.

Those skilled in the art will appreciate that various adaptations andmodifications of the just-described preferred embodiments can beconfigured without departing from the scope and spirit of the invention.Therefore, it is to be understood that, within the scope of the appendedclaims, the invention may be practiced other than as specificallydescribed herein.

1. A mechanism moving a slider toward a track on a disk surface in ahard disk drive, to minimize track mis-registration, comprising: meansfor moving said slider parallel to said disk surface toward said track,when said disk surface is flat, by an actuator arm moving said slider bya lever action through a principal axis with said slider aligned at abias angle; wherein a read-write head is encapsulated in said sliderfacing said rotating disk surface about a radial center in a hard diskdrive; wherein said read-write head is communicatively coupled with saidrotating disk surface to communicatively access said track; and meansfor radially moving said slider toward said track when said disk surfaceis bent, by said lever action through said principal axis at said biasangle causing said slider to move radially toward said track, when saiddisk surface is bent.
 2. The mechanism of claim 1, wherein the means formoving said slider parallel said disk surface arm further comprisesmeans for said actuator arm moving, through a flexure, said slidermounted to said flexure at a second bias angle to said principal axis;wherein the means for radially moving said slider further comprising:said flexure responding as said disk surface is bent, through saidsecond bias angle, causing said slider to move radially toward saidtrack.
 3. The mechanism of claim 2, wherein said flexure is mounted tosaid actuator arm at said second bias angle.
 4. The mechanism of claim3, wherein at least two welds mount said flexure to said actuator arm atsaid second bias angle.
 5. The mechanism of claim 4, wherein at leasttwo welds mount said flexure to a load beam coupled to said actuator armat said second bias angle.
 6. The mechanism of claim 2, wherein saidslider is mounted to said flexure at said second bias angle.
 7. Themechanism of claim 2, wherein said second bias angle is between one-halfdegree and three degrees.
 8. The mechanism of claim 7, wherein saidsecond bias angle is between three-quarters degree and five-halvesdegrees.
 9. The mechanism of claim 1, wherein said actuator arm includessaid slider attached through a flexure to a load beam and wherein saidload beam is aligned to said principal axis at said bias angle.
 10. Themechanism of claim 9, wherein said actuator arm includes an extendedbase plate with a bent edge attaching to a bent edge of said load beamto create said load beam aligned to said principal axis at said biasangle.
 11. The mechanism of claim 9, wherein said actuator arm includesa mounting surface base plate with a bent edge attaching to said loadbeam to create said load beam aligned to said principal axis at saidbias angle.
 12. The mechanism of claim 11, wherein said actuator armincludes said mounting surface base plate with said bent edge attachingto a bent edge of said load beam to create said load beam aligned tosaid principal axis at said bias angle.
 13. The mechanism claim 12,wherein said actuator arm includes said mounting surface base plate withsaid bent edge attaching through a connection beam to said bent edge ofsaid load beam to create said load beam aligned to said principal axisat said bias angle.
 14. The mechanism of claim 9, wherein said actuatorarm includes an extended base plate with a bent edge attaching to saidload beam to create said load beam aligned to said principal axis atsaid bias angle.
 15. The mechanism of claim 14, wherein said actuatorarm includes said extended base plate with said bent edge attaching to abent edge of said load beam to create said load beam aligned to saidprincipal axis at said bias angle.
 16. The mechanism of claim 15,wherein said actuator arm includes said extended base plate with saidbent edge attaching through a connection beam to said bent edge of saidload beam to create said load beam aligned to said principal axis atsaid bias angle.
 17. The mechanism of claim 1, wherein said actuator armis coupled to said load beam via a first finger and a second finger;wherein said first finger flexes differently from said second fingerwhen said disk surface is bent; and wherein the means for radiallymoving said slider further comprises said first finger flexingdifferently from said second finger flexing causing said slider to moveradially toward said track, when said disk surface is bent.
 18. Themechanism of claim 17, wherein a width of said first finger differs froma width of said second finger to cause said first finger to flexdifferently from said second finger.
 19. The mechanism of claim 17,wherein a shape of said first finger differs from a shape of said secondfinger to cause said first finger to flex differently from said secondfinger.
 20. A method of moving a slider toward a track on a disk surfacein a hard disk drive, to minimize track mis-registration, comprising thesteps of: moving said slider parallel to said disk surface toward saidtrack, when said disk surface is flat, with an actuator arm moving by alever action through a principal axis with said slider aligned at a biasangle; wherein a read-write head is encapsulated in said slider facingsaid rotating disk surface about a radial center in a hard disk drive;wherein said read-write head is communicatively coupled with saidrotating disk surface to communicatively access said track; wherein saidmethod further comprising the step of: radially moving said slidertoward said track, when said disk surface is bent, by said lever actionthrough said principal axis at said bias angle causing said slider tomove radially toward said track, when said disk surface is bent.
 21. Themethod of claim 20, wherein the step moving said slider parallel saiddisk surface arm further comprising the step of: said actuator armmoving, through a flexure, said slider mounted to said flexure at asecond bias angle to said principal axis; wherein the step radiallymoving said slider further comprising the step of: said flexureresponding as said disk surface is bent, through said second bias angle,causing said slider to move radially toward said track.
 22. The methodof claim 21, wherein said flexure is mounted to said actuator arm atsaid second bias angle.
 23. The method of claim 22, wherein at least twowelds mount said flexure to said actuator arm at said second bias angle.24. The method of claim 23, wherein said at least two welds mount saidflexure to a load beam coupled to said actuator arm at said second biasangle.
 25. The method of claim 21, wherein said slider is mounted tosaid flexure at said second bias angle.
 26. The method of claim 21,wherein said second bias angle is between one-half degree and threedegrees.
 27. The method of claim 26, wherein said second bias angle isbetween three-quarters degree and five-halves degrees.
 28. The method ofclaim 20, wherein said actuator arm includes said slider attachedthrough a flexure to a load beam; wherein said load beam is aligned tosaid principal axis at said bias angle.
 29. The method of claim 28,wherein said actuator arm includes an extended base plate with a bentedge attaching to a bent edge of said load beam to create said load beamaligned to said principal axis at said bias angle.
 30. The method ofclaim 28, wherein said actuator arm includes a mounting surface baseplate with a bent edge attaching to said load beam to create said loadbeam aligned to said principal axis at said bias angle.
 31. The methodclaim 30, wherein said actuator arm includes said mounting surface baseplate with said bent edge attaching to a bent edge of said load beam tocreate said load beam aligned to said principal axis at said bias angle.32. The method claim 31, wherein said actuator arm includes saidmounting surface base plate with said bent edge attaching through aconnection beam to said bent edge of said load beam to create said loadbeam aligned to said principal axis at said bias angle.
 33. The methodof claim 28, wherein said actuator arm includes an extended base platewith a bent edge attaching to said load beam to create said load beamaligned to said principal axis at said bias angle.
 34. The method ofclaim 33, wherein said actuator arm includes said extended base platewith said bent edge attaching to a bent edge of said load beam to createsaid load beam aligned to said principal axis at said bias angle. 35.The method of claim 34, wherein said actuator arm includes said extendedbase plate with said bent edge attaching through a connection beam tosaid bent edge of said load beam to create said load beam aligned tosaid principal axis at said bias angle.
 36. The method of claim 20,wherein said actuator arm is coupled to said load beam via a firstfinger and a second finger; wherein said first finger flexes differentlyfrom said second finger, when said disk surface is bent; wherein thestep radially moving said slider further comprising the step of: saidfirst finger flexing differently from said second finger flexing causessaid slider to move radially toward said track, when said disk surfaceis bent.
 37. The method of claim 36, wherein the width of said firstfinger differs from the width of said second finger to cause said firstfinger flexing differently from said second finger.
 38. The method ofclaim 36, wherein the shape of said first finger differs from the shapeof said second finger to cause said first finger flexing differentlyfrom said second finger.
 39. A head gimbal assembly supporting themethod of claim 20, comprising, for each of the steps of claim 20, ameans for implementing said step.
 40. Said actuator arm of claim 20,comprising, for each of the steps of claim 20, a means for implementingsaid step.
 41. An actuator supporting the method of claim 20,comprising, said actuator arm; and further comprising, for each of thesteps of claim 20, a means for implementing said step.
 42. Said harddisk drive of claim 20, comprising, for each of the steps of claim 20, ameans for implementing said step.
 43. A mechanism moving a slider towarda track on a disk surface in a hard disk drive, to minimize trackmis-registration, comprising: means for moving said slider parallel tosaid disk surface toward said track, by an actuator arm moving a coupledload beam via a first finger and a second finger; wherein said firstfinger flexes differently from said second finger when said disk surfaceis bent; wherein said slider is coupled to said load beam; and wherein aread-write head is encapsulated in said slider facing said rotating disksurface about a radial center in a hard disk drive; wherein saidread-write head is communicatively coupled with said rotating disksurface to communicatively access said track; and means for radiallymoving said slider toward said track, when said disk surface is bent, bysaid first finger flexing differently from said second finger flexing tocause said slider to move radially toward said track, when said disksurface is bent.
 44. The mechanism of claim 43, wherein the width ofsaid first finger differs from the width of said second finger to causesaid first finger flexing differently from said second finger.
 45. Themechanism of claim 43, wherein the shape of said first finger differsfrom the shape of said second finger to cause said first finger flexingdifferently from said second finger.
 46. The mechanism of claim 43,wherein said first finger couples to a top side of said load beam; andwherein said second finger coupled to a bottom side of said load beam.47. The mechanism of claim 46, wherein said first finger is formed by afirst connection beam coupling said actuator arm to said load beam. 48.The mechanism of claim 43, wherein the means for moving said sliderparallel said disk surface arm further comprising: means for saidactuator arm moving, through a flexure, said slider mounted to saidflexure at a second bias angle to a principal axis; wherein the meansfor radially moving said slider further comprising: said flexureresponding as said disk surface is bent, through said second bias angle,causing said slider to move radially toward said track.
 49. Themechanism of claim 48, wherein said flexure is mounted to said actuatorarm at said second bias angle.
 50. The mechanism of claim 49, wherein atleast two welds mount said flexure to said actuator arm at said secondbias angle.
 51. The mechanism of claim 50, wherein at least two weldsmount said flexure to said load beam coupled to said actuator arm atsaid second bias angle.
 52. The mechanism of claim 48, wherein saidslider is mounted to said flexure at said second bias angle.
 53. Themechanism of claim 48, wherein said second bias angle is betweenone-half degree and three degrees.
 54. The mechanism of claim 53,wherein said second bias angle is between three-quarters degree andfive-halves degrees.
 55. A method of moving a slider toward a track on adisk surface in a hard disk drive, to minimize track mis-registration,comprising the steps of: moving said slider parallel to said disksurface toward said track, when said disk surface is flat, by anactuator arm moving a coupled load beam via a first finger and a secondfinger; wherein said first finger flexes differently from said secondfinger when said disk surface is bent; wherein said slider is coupled tosaid load beam; and wherein a read-write head is encapsulated in saidslider facing said rotating disk surface about a radial center in a harddisk drive; wherein said read-write head is communicatively coupled withsaid rotating disk surface to communicatively access said track; andradially moving said slider toward said track, when said disk surface isbent, by said first finger flexing differently from said second fingerflexing to cause said slider to move radially toward said track, whensaid disk surface is bent.
 56. The method of claim 55, wherein the widthof said first finger differs from the width of said second finger tocause said first finger flexing differently from said second finger. 57.The method of claim 55, wherein the shape of said first finger differsfrom the shape of said second finger to cause said first finger flexingdifferently from said second finger.
 58. The method of claim 55, whereinsaid first finger couples to a top side of said load beam; and whereinsaid second finger coupled to a bottom side of said load beam.
 59. Themethod of claim 58, wherein said first finger is formed by a firstconnection beam coupling said actuator arm to said load beam.
 60. Themethod of claim 55, wherein the step moving said slider parallel saiddisk surface arm further comprising the step: said actuator arm moving,through a flexure, said slider mounted to said flexure at a second biasangle to a principal axis; wherein the step radially moving said sliderfurther comprising the step: said flexure responding as said disksurface is bent, through said second bias angle, causing said slider tomove radially toward said track.
 61. The method of claim 60, whereinsaid flexure is mounted to said actuator arm at said second bias angle.62. The method of claim 60, wherein at least two welds mount saidflexure to said actuator arm at said second bias angle.
 63. The methodof claim 62, wherein at least two welds mount said flexure to said loadbeam coupled to said actuator arm at said second bias angle.
 64. Themethod of claim 60, wherein said slider is mounted to said flexure atsaid second bias angle.
 65. The method of claim 60, wherein said secondbias angle is between one-half degree and three degrees.
 66. The methodof claim 60, wherein said second bias angle is between three-quartersdegree and five-halves degrees.